rcu: rcupdate.h: Get rid of Sphinx warnings at rcu_pointer_handoff()

The code example at rcupdate.h currently produce lots of warnings:

	./include/linux/rcupdate.h:572: WARNING: Unexpected indentation.
	./include/linux/rcupdate.h:576: WARNING: Unexpected indentation.
	./include/linux/rcupdate.h:580: WARNING: Block quote ends without a blank line; unexpected unindent.
	./include/linux/rcupdate.h:582: WARNING: Block quote ends without a blank line; unexpected unindent.
	./include/linux/rcupdate.h:582: WARNING: Inline literal start-string without end-string.

This commit therefore changes it to a code-block.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>